1. Field of the Invention
The present invention relates to a method for manufacturing a functional device, more particularly, to a method for manufacturing a functional device preferably having a substantially flat reflective surface or a V-shaped groove at an angle of 45 degrees relative to a (100)-plane of a silicon substrate on the substrate by utilizing a wet etching.
2. Description of the Related Art
Optical disk devices represented by CD-ROMs (Compact Disk-Read Only Memory), MOs Magneto-Optical disk) and DVDs (Digital Versatile Disk) which reproduce information by the use of laser beams having a predetermined wavelength are widely used. An optical head which forms a major component of an optical head apparatus includes a laser diode and a photodiode integrated thereon.
FIG. 1 is a perspective view showing a schematic configuration of an IOU (Integrated Optical Unit) 20, which is a type of functional device, a main portion of the optical head. The IOU 20 integrates a light-receiving element 24 that receives reflected light beam, a laser diode 25 that deflects the reflected light beam by 90 degrees by using a 45-degree-mirror 26 to emit an irradiated light beam 22, and the like on a silicon substrate 20.
FIG. 2 is a sectional view showing the laser diode 25 and the 45-degree-mirror 26. The irradiated light beam 22 emitted by the laser diode 25 mounted on the silicon substrate 21 is reflected by the 45-degree-mirror 26 to be deflected upward and is guided to an optical disk through a lens system (not shown). The 45-degree-mirror 26 that reflects the irradiated light beam 22 is fabricated by etching on a silicon substrate 21 and depositing a reflective film made of Al, Au or the like on a surface of the 45-degree-mirror 26.
Although the IOU 20 can be obtained in compact integrated form as a result, it has a serious problem in that a high cost is required for processing the silicon substrate 21 used therefor. The reason is that a (111)-plane is generally employed as the reflective surface of the 45-degree-mirror 26 and this requires a wafer having special specifications that are well known in the art by chemical companies that fabricate wafers.
A cut surface of a wafer sold as a xe2x80x9c(100) silicon waferxe2x80x9d sliced from a silicon ingot is normally referred to as xe2x80x9c(100)-planexe2x80x9d.
As described, for example, by Kurt E. Peterson, Proceedings of the IEEE, vol. 70, No. 5, pp. 424 (1982) (which is incorporated herein by reference in its entirety), it is known that when the (100)-plane of the silicon substrate 21 is etched, a (111)-plane appears at an angle of exactly 54.74 degrees to the (100)-plane on the section of the silicon substrate 21 as shown in FIG. 3A. Because such a plane cannot be used as a 45-degree-mirror as it is, as shown in FIG. 3B, a silicon ingot 31 is sliced at an angle to obtain the (100) silicon wafer which is 9.74 degrees off, that is to make the angle exactly 45 degrees. The wafer is then etched to obtain a 45-degree-surface. It is difficult to control the inclination at the angle of 9.74 degrees accurately when the silicon ingot 31 is sliced. For example, when a round-bar ingot is sliced at an angle, the sliced part will have a elliptic configuration. Because the sliced wafer is difficult to process using a normal exposure apparatus if it remains in the elliptic configuration, a shaping operation is generally required to shape it into a circular configuration. Such a shaping operation involves much time and labor, resulting in a high cost. Further, the number of wafers available from one ingot 31 is small when they are obtained by slicing the wafers at an angle, and this also leads to a high cost. Presently, the cost of the 45 degree (100) wafer is at a level three or four times higher than that of normal (100) silicon wafers.
Attempts have been made to form the 45-degree-surface using a (100) silicon wafer which is less expensive. Specifically, a silicon substrate is patterned in a  less than 100 greater than  direction as shown in FIG. 4 and is etched using an anisotropic etchant to form a surface at an angle of 45 degrees to the (100)plane of the silicon substrate, ie., the 45-degree-surface constituted by a (110)-plane.
In practice, however, the section becomes vertical instead of being at 45 degrees, as shown in FIG. 5, depending on the etchant used. Even when a 45-degree-surface is obtained, the surface has significant irregularities thereon such that it is difficult to achieve the desired RMS (Root Mean Square) of such a 45-degree-surface (i.e., 30 nm or less). Under such circumstances, there has been no report on successful formation of a smooth surface which can be used as a reflective surface Under some circumstances, a RMS of 30 nm or less is very important, for example, to read information on particular types of optical disks, such as DVD disks.
Backlund et al, Optical Planes and Reflectors, Anisotropically Etched in Silicon. 7th Int. Conf. On Solid-State Sensors and Actuator (Transducers 93) Yokohama, Japan, 1993, pp. 1031-1033 (which is incorporated herein by reference in its entirety) also describes chemical etching of silicon.
As described above, conventional functional devices and methods for manufacturing thereof have had a problem in that a 45-degree-surface has required a very high cost. Further, efforts toward the use of wafers at low cost have been unsuccessful in achieving desired configurations and operations.
The present invention has been made in view of the above-mentioned circumstances and intended to solve the above-mentioned problems. In particular, one object of the invention to provide a method for manufacturing a functional device with a 45-degree-surface formed by a (110)-plane using a (100) silicon wafer which is at a low cost.
The present invention provides a method for manufacturing a functional device including preparing a substrate made of a (100) silicon forming a pattern along a  less than 100 greater than  direction of the (100) silicon, and etching with an anisotropic etchant using the pattern while applying an ultrasonic wave.
The present invention also provides a method for manufacturing a semiconductor device including preparing a substrate made of a (100) silicon, forming a pattern along a  less than 100 greater than  direction of the (100) silicon and etching with an anisotropic etchant and a surface active agent.
Here, the surface formed by the etching step preferably has an angle of 45 degrees against the surface of the (100) silicon. The anisotropic etchant may include a surface active agent. The method may further comprise a step for coating a reflective material on the surface formed by the etching step.
Furthermore, the present invention provides a functional device including a (100) silicon, a pattern formed on the (100) silicon having a slanted surface along a  less than 100 greater than  direction, wherein an RMS (Root Mean Square) of the slanted surface is 50 nm or less.
The present invention yet further provides an optical head apparatus, comprising, a light source that irradiates a light beam along a first direction, a reflector that reflects the light beam to a second direction, and an objective lens that receives the light beam from the reflector and condenses the light beam onto an optical recording medium, wherein the reflector is formed on a slanted surface along a  less than 100 greater than  direction of a (100) silicon, and a RMS (Root Mean Square) of the slanted surface is 50 nm or less.
Further objects features and advantages of the invention will become apparent from the detailed description of preferred embodiments that follows.